Conversions are common in London because the land is already built on. The commercial problem is that every conversion starts with an existing building whose real dimensions, structure and services are unknown until somebody measures them. As quantity surveyors in London, most of the disputes we are asked to unpick on conversions begin with a quantity taken from the wrong drawing.
A sales plan, a historic drawing and a measured survey are three different documents. Only one of them is safe to price from.
Why the plan you were given is not a measurement
Marketing plans are produced to sell space and are usually drawn to an approximate outline. Historic drawings record what was intended at the time, not what was built or what has been altered since. Neither is a record of the structure behind the finishes.
Pricing from either one transfers a measurement risk to whoever prices it, and on a fixed price contract that party will price the risk or exclude it. Both outcomes cost you money.
What a measured survey actually gives you
A measured survey gives dimensions you can rely on, floor to floor heights, wall thicknesses and the position of things that cannot move. On a conversion those last items decide the layout, because a stair, a riser or a structural wall in the wrong place removes a unit from the scheme.
It is cheap relative to what it prevents. A scheme appraised on an assumed net to gross ratio and then surveyed can lose a unit, and one unit usually costs more than every survey on the project put together.
The structure is the real unknown
Load paths in an existing building are frequently undocumented. Previous alterations may have removed or added support without a record. Floor construction varies between parts of the same building where it was extended at different times.
Opening up is the only way to answer these questions, and opening up is itself a cost with a programme. Where it has not happened, the cost plan should say so on its face rather than carry a quietly optimistic rate. That principle is set out in measuring an existing building before you price it.
Services, risers and the things nobody drew
Existing drainage runs, incoming services, riser positions and their capacity decide whether a proposed layout is buildable. On a change of use the demand profile changes completely, and the existing incoming supply may not be adequate for the new one.
Upgrading an incoming supply involves a third party with its own timetable and its own charges. Treat it as a long lead item in the programme, not as a line in the mechanical package.
Net to gross is where conversions are won or lost
On a conversion the saleable area is what is left after the structure, the cores, the risers and the circulation have taken their share. Those are largely fixed by the building that already exists, so the ratio cannot be improved by drawing it differently.
An appraisal built on a net to gross figure borrowed from a new build scheme will overstate the saleable area, and the overstatement falls straight through to the profit. On a small scheme it can be the whole profit.
The survey settles it. Until it exists, the appraisal should carry the area as provisional and say so, rather than presenting an assumption as a measurement.
Fire, access and compliance on a change of use
Changing what a building is used for changes what the regulations require of it. Means of escape, compartmentation, access and acoustic separation between dwellings are the usual items, and in an existing building each of them is harder than in a new one.
These are matters for a designer and for building control on the specific building, and the detail depends on the authority involved. For a budget the point is simply that they are not incidental, and that the cost of achieving them in an existing structure is discovered by survey rather than assumed.
Measuring for the contract, not just for the design
Once the survey exists, the quantities taken from it should be recorded with the drawing revision they came from, what was included and what was assumed. On a conversion the assumptions are the valuable part, because they are what you will argue about when the building turns out differently.
A schedule that labels measured items, model derived items and assumed items separately is worth more later than a tidier one that hides the difference. See quantities and take-off.
What this means for your appraisal
Price the survey work, the opening up and the time they take before you price the conversion. Treat any area taken from a sales plan as provisional until it is measured, and keep the appraisal flagged as provisional while it rests on that.
Where a scheme only works on the assumed areas, it does not yet work. It is a scheme waiting for a survey to confirm or remove it.
The order that protects you is straightforward. Measure the building, test the layout against what the structure allows, price the scheme from the measured quantities, then decide. Each step costs a fraction of the one after it, and each can stop the project before the expensive commitment is made.
The order that costs money is the reverse: commit, design, discover. On a conversion that sequence does not usually produce a worse building. It produces the same building at a price nobody agreed to.
If the building is occupied while you survey it, factor that in as well. Access to tenanted parts is negotiated rather than assumed, and a survey that covers eighty per cent of a building leaves the remaining twenty per cent as a risk sitting in your contingency rather than in your drawings.
Converting an existing building?
Send us what you have, even if it is only a sales plan. We will tell you what has to be measured before the appraisal means anything, and what that is likely to cost.